The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Hybrid and electric vehicle systems use high voltage systems that provide electric power for traction motors and other electric machines. A high voltage system includes a high voltage DC electric power source, e.g., a battery that electrically connects to the traction motor via a power inverter. The power inverter converts DC electric power to AC electric power to drive the traction motor, and preferably converts AC electric power to DC electric power for charging the battery. The high voltage DC electric power source electrically connects to the power inverter via a positive high-voltage bus (HV+) and a negative high-voltage bus (HV−). The other electric machines using high voltage power are also electrically connected to the positive high-voltage bus and the negative high-voltage bus. The positive high-voltage bus and the negative high-voltage bus are electrically isolated from a chassis ground.
Electric machines, e.g., traction motors, include rotors that rotate in response to alternating current (AC) electric power applied to associated stators. The rotors can be mechanically coupled to power transmission devices to provide tractive power to a driveline of a vehicle.
Known voltage source inverter circuits and associated control circuits can convert direct current (DC) electric power originating from a high-voltage energy storage device to alternating current (AC) electric power to generate tractive power in response to operator requests. Known inverter circuits include MOSFET and IGBT switch devices. Electric load requirements can include presently occurring electric loads and battery charging to meet future electric loads.
A voltage source inverter uses a floating DC bus setup wherein the DC input voltages are configured to float with reference to a chassis ground. This floating can be controlled by using balanced high impedance resistors that connect high voltage DC buses to the chassis ground. Capacitors can be electrically connected in parallel with the high impedance resistors to provide low impedance shunt paths for high frequency electric noise currents. In one embodiment, half the DC bus voltage is applied across a positive electric power bus (HV+) to the chassis ground, and half the DC bus voltage is applied across a negative electric power bus (HV−) to the chassis ground. An AC side of the voltage source inverter floats with reference to the chassis ground.
There are two potential faults associated with loss of ground isolation. One fault is an AC loss of isolation, wherein one of the phases of the AC side is shorted to chassis ground. Another fault is a DC loss of isolation wherein one of the positive electric power bus (HV+) and the negative electric power bus (HV−) is shorted to the chassis ground or has a reduced impedance relative to the chassis ground. When an AC loss of isolation fault occurs on one of the phases of the AC side of the voltage source inverter, AC current associated with activation and deactivation of the switch devices of the inverter flows through the capacitors that provide the low impedance shunt paths for high frequency electric noise currents. In the event of a ground isolation fault, AC current associated with one of the phase voltages of the inverters from the positive electric power bus (HV+) to the negative electric power bus (HV−) can cause excessive electric current flow to the capacitors that provide the low impedance shunt paths for high frequency electric noise currents. AC current in excess of the capacity of the capacitors can cause capacitor faults and associated inverter damage.
A known solution to detect an AC loss of isolation fault includes measuring electric current through each of the electric cables associated with the phases of the AC side of the voltage source inverter and arithmetically summing them. In an ideal system operating without a fault, the sum of the measured electric currents is zero at any selected point in time. When a ground isolation fault is present, the sum of the measured electric currents is a value other than zero. Issues associated with this solution include measurement errors associated with signal outputs from the electric current sensing devices, which can be cumulative. This can cause an error in the overall current signal. Furthermore, a phase current sensor can have bandwidth/step response limitations due to magnetic and electrical response characteristics and sensor saturation. Thus a fault may not be detected depending on the timing of the sample measurement in relation to switching events associated with the inverter. Furthermore, a ground isolation fault and associated change in impedance can include a resonance element, with data sampling occurring at or near a zero crossing. Thus, any fault current may be aliased out. Furthermore, when impedance approaches zero, any current oscillation may be dampened out in less than the sampling time. Thus the fault current may not be measured.
A known solution to detect DC loss of isolation includes measuring voltage between the positive electric power bus (HV+) and the negative electric power bus (HV−), and measuring voltage between the negative electric power bus (HV−) and the chassis ground, and calculating a voltage ratio based thereon. One calculation for the voltage ratio for loss of isolation detection is 2*(voltage measurement between HV− to chassis ground)/(voltage measurement between HV+ to HV−). Issues associated with this known solution include signal measurement errors that need to be accounted for and are often cumulative. The effect of signal measurement errors is that there can be a lack of separation between a “must detect” and a “must not detect” threshold, which can lead to false fault detection. Furthermore, known DC voltage sensors can have bandwidth and response time measurement limitations. Thus a fault may not be detected. Furthermore, the timing of the measurement sample relative to a fault and an associated switching event may result in a fault not being recorded. Furthermore, fault impedance may include a resonance element, with data sampling occurring at or near the zero crossing. Thus, a fault current may be aliased out. Furthermore, when a switching period associated with the inverter is near a 50% duty cycle, an average voltage may still be near an expected level. Thus, a fault voltage may not be measured.